The present invention relates to a liquid crystal display circuit for driving a liquid crystal of a portable information terminal or the like, and move particularly to a liquid crystal display circuit capable of changing the tone of each pixel stepwise.
Conventionally,. in order to make the tone of each pixel have gradation, as shown in FIG. 9(a), there is known a means in which for a common signal COM 1, eight pulse signals of PW=1 to 8 with different pulse widths are prepared as segment signals for driving pixels, and a driving time is finely adjusted thereby. The means is also called a PWM (pulse width modulation) system in the sense that the pulse width of a segment signal is made variable. In this PWM system, as the variable width of the pulse width is made fine, the precision of the gradation becomes excellent, while there occurs a problem in that as the variable width is made fine, the frequency of a control clock becomes high.
In addition to the control of the pulse width, as shown in FIG. 9(b), there is also a system using frame modulation as well in which the way of outputting a segment pulse is changed to x1, x2, x3, and x4 for each of a pair of frames F1, F2, F3, and F4 of a display screen so that tone control is made. In this manner, in order to achieve fine tone control, it is desirable to use both the pulse width modulation and the frame modulation, and this has been common as a system of gradation control.
As is understood from FIG. 9(b), the conventional method of frame modulation is performed such that control is made as to whether or not a signal prepared as a segment signal is output in one pair of continuous frames, and such an effect can be expected that when the signal is output for all frames (the case of x4 in the example of FIG. 9(b)), the pixel becomes dense, and in the case where the signal is output for once every four frames as in the example of x1 of FIG. 9(b), the pixel becomes pale.
However, since conventional frame modulation is a method of determining whether or not a segment signal is output, there has been a problem in that when an attempt is made to control gradation stepwise, it lacks continuity.
This will be described with reference to FIG. 10. In a timing diagram shown in FIG. 10(a), when the pulse width of a segment signal to be output is 1 (PW=1), since it is possible to effect control such that one pulse is output or two pulses are output (26 to 29) in the continuous four frames (F1, F2, F3, and F4), when summing is made in the four frames, the fineness, that is, the sum of segment driving times in the four frames is continuous as 1, 2, 3, and 4. On the other hand, in the case of FIG. 10(b), since the pulse width of a segment signal to be output is 3 (PW=3), the fineness becomes discrete values such as 3, 6, 9, and 12.
When continuity of gradation control is considered, in the conventional system, as described above, since gradation setting becomes discrete and unevenness occurs, there occurs a number of levels which can not be set through the driving time of a total of the four frames. This means that since control intervals are irregular in the case where gradation control is made stepwise, tone control becomes partially impossible.
An object of the present invention is to provide a liquid crystal display circuit which is improved and is capable of solving the foregoing problem in a liquid crystal display device in which tone control of a pixel is made by using both pulse width modulation and frame modulation.
In order to solve the foregoing problem, according to the present invention, in a liquid crystal display circuit enabling gradation display of pixel by using both pulse width modulation in which drive pulse width for each segment is changed stepwise and frame modulation in which the manner of outputting a drive pulse is changed stepwise for each of one pair of frames of a display screen, control is made for each of the frames of the display screen as to whether or not a pulse width of a drive signal of each pixel is to be increased by a minimum fine adjustment width, so that the total density of the one pair of frames has continuity and unevenness does not occur in gradation setting.
In the above structure the device, it is also possible to structure such that it is determined by a value of lower bits of a gradation pallet whether or not the width of the drive signal of each pixel is increased by the minimum fine adjustment width.